Convert ds field formatters to field formatter plugins

/**
 * Implements hook_field_formatter_info().
 */
function ds_field_formatter_info() {

...

  return $formatters;
}

/**
 * Implements hook_field_formatter_view().
 */
function ds_field_formatter_view(EntityInterface $entity, $field, $instance, $langcode, $items, $display) {
...
}